Patination of copper



United States Patent U.S. Cl. 148-624 8 Claims ABSTRACT OF THEDISCLOSURE A method is described for producing a synthetic patina ofbrochantite and malachite on copper by immersion of the copper in anaqueous acidic solution at ordinary temperatures and pressures in whichthere are dissolved 2.5-10 g./l. of potassium chlorate and 0.520 g./l.of copper sulfate.

BACKGROUND OF THE INVENTION Field of the invention When copper and itsalloys are exposed to the natural atmosphere, their surface colors willgradually change according to the copper composition and the atmosphericconditions. In the natural weathering of copper, the color changesgradually from the bright metallic pink of the new metal through redbrown, dark brown, ebony, gray and gray-green to the ultimate blue-greenpatina. The color change of some copper base alloys follows an analogouspattern, though the rate of change and color produced may differ fromthose on unalloyed copper The gradual color change of the cupreoussurface is due to the slow formation of a relatively thin surface layerof copper compounds produced by the chemical reaction of the cupreoussurface with atmospheric constituents, such as, oxygen, carbon dioxide,hydrogen sulfide, sulfur dioxide, and sodium chloride in the presence ofatmospheric humidity, dew and rain. Depending on the composition of thecupreous surface, the climate and certain atmospheric contaminants, thisnatural patina may appear on the surface only after exposure to theelements for decades or centuries.

In 1929, Vernon established patina to be a basic sulfate of the samecomposition as the mineral brochantite [CuSO -3Cu(OH) An exception tothis is a product from a purely marine atmosphere, in which the basicchloride atacamite [CuCl -3Cu(OH) predominates. Where urban and marineconditions coincide, basic sulfate predominates over basic chloride.Basic carbonate, malachite [CuCO -Cu(OH) only appears to a minor extent.

The colors of the natural patina are very pleasing in appearance and aredesirable in many architectural and decorative applications. The lengthof time required and the necessity for certain atmospheric and climaticconditions, however, render dependence on the natural process forcommerical use impractical. Therefore, many artificial methods usingchemicals or electrolytic processes were proposed to provide thecupreous surface with a pleasing and stable patina finish in a shorterlength of time.

It has been proposed heretofore to develop a patina coloration oncupreous surface artificially by immersing such surface in various saltsolutions. The chemicals, or mixtures of chemicals, using such solutionsare intended to react relatively quickly with the cupreous surface toprovide a surface layer of adhering, insoluble copper compounds. Thecolor of this surface layer may vary considerably depending on thechemicals used and the "ice composition of the cupreous surface.Moreover, repeated applications are necessary with many of these priorprocesses, in order to obtain sufiicient depth of color.

Of the various methods developed, the electrolytic method of Vernon andthe CABRA (The Copper & Brass Research Association in America) sprayprocess have been considered to be the most promising methods. In theVernon anodic method, an electrolyte consisting of magnesium sulfate,magnesium hydroxide, and potassium bromate in water is used for thepatinating process. The deposit obtained, still requires aging to fullydevelop the patina. Because it is an electrolytic method, patinatingtreatment of large surface areas becomes difiicult if not impossible.The CABRA process requires a thoroughly clean and oxide-free surfacebefore the application of an aqueous coloring solution consisting ofammonium sulfate, copper sulfate, and ammonia. The amount of ammoniaused must be measured exactly since the ratio of ammonia to water iscritical. The solution is spraycoated on the surface and allowed to drybefore a second application can be made. The spraying and drying cyclemust be repeated five or six times and often up to eight cycles arerequired. A properly sprayed finish appears glassy and requiresweathering for the full patina to develop under suitable atmosphericconditions.

SUMMARY OF THE INVENTION It is the object of this invention to overcomethe disadvantage of the prior methods by providing a method and acoating composition which have the advantage of greater simplicity andversatility and are capable of providing a synthetic patina of the basicsulfate and carboriate which is chemically and physically the same asthe one produced by the natural aging process.

According to the present invention, no spray or electroplating equipmentis required because the patina is applied by immersing the copper intothe solution at ordinary temperature and pressure. Furthermore, only asingle application is required and no fixing solution is necessar roadlystated the method of imparting a decorative patina finish to a cupreoussurface in accordance with this invention comprises covering saidsurface at ordinary temperature and pressure with a layer of an aqueousacidic solution in which there are dissolved 2.5-10 g./l. of potassiumchlorate, 0.5-20 g./l. of copper sulfate, up to 10 g./l. sodium sulfateand up to 2 g./l. of sodium bicarbonate. The solution is made acidicwith sulfuric acid. The amount of each ingredient to be used in thesolution can be varied within a wide range depending upon the type offinish desired.

We have found by using a solution of this invention a pleasing syntheticpatina finish can be imparted to cupreous surfaces without the necessityof repeated applications required by prior methods. The process involvesimmersion of the copper into an aqueous acidic solution at ordinarytemperature and pressure for a period of days.

X-ray diffraction analysis shows that a cupreous oxide (Cu O) layer isformed on the copper initially, generally within 12 hours. After a fewdays the cupreous oxide is gradually transformed into an outer layer ofbrochantite [CuSO -3Cu(OH) or malachite [CuCO -Cu(OH) depending on theparticular solution used.

The solution is rendered acidic by the addition of an acid such assulfuric acid until the proper pH is attained. There may be othersubstances present to obtain special advantage in rate and nature ofcrystal growth. Copper sulfate is essential to the second stage of theprocess where the cupreous oxide is slowly transformed to basicsulfate-brochantite. Sodium bicarbonate is essential to the second stageof the process where the cupreous oxide is slowly transformed to basiccarbonate-malachite.

In order to further illustrate this invention, specific examples aredescribed below.

EXAMPLE 1 A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H 3.2 Sodiumsulfate, Na SO Sulfuric acid to pH 3 The surface of the copper to betreated is cleaned with an abrasive pad to remove oxide, oil and dust.The cleaned Copper article then is immersed in the solution at ordinarytemperature and pressure (by which is meant ambient room temperature andatmospheric pressure). Development of the synthetic patina beginsimmediately after immersing the copper into the aqueous acidic solution.After a few days the surface presents a vivid green appearance as theouter layer has been transformed into brochantite. The copper is removedon the twenty-first day and washed in a water bath.

EXAMPLE 2 A solution having the following composition was prepared.

G./l. Potassium chlorate, K010 l 5 Copper sulfate, CuSO -5H O Sulfuricacid, H 50 Treatment of a copper article in this solution is the same asin Example 1 but the exposure to the aqueous acidic solution is for nine(9) days at which time the outer layer of the copper has beentransformed into brochantite.

EXAMPLE 3 A solution having the following composition was prepared.

G./l. Potassium chlorate, K010 5 Copper sulfate, CuSO -5H O 1.25 Sodiumsulfate, Na SO 2 Sulfuric acid, H 50 Treatment of a copper article inthis solution is the same as in Example 1 but the pH is adjusted to 3.5and the exposure to the aqueous acidic solution is for twenty (20) daysat which time the outer layer of the copper has been transformed intobrochantite.

EXAMPLE 4 A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H O 1.6 Sodiumsulfate, Na SO 2.5

Sulfuric acid, H 80 Treatment of a copper article in this solution isthe same as in Example 1 but the exposure to the aqueous acidic solutionis for twenty (20) days at which time the outer layer of the copper hasbeeen transformd into brochantite.

In Examples 1 to 4 the composition may be varied in accordance with thetable below:

G./l. Potassium chlorate, KClO 2.5-1O Copper sulfate, CuSO -5H O 0.5-20

Sodium sulfate, Na SO Up to 10 Sulfuric acid, H SO To adjust thesolution to pH of 2.0-4.0 in which the cupreous surface is exposed for9-21 days and is transformed into brochantite CuSO '3Cu (OH) 4 EXAMPLE 5A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H O 1 Sodiumbicarbonate, NaHCO l Sulfuric acid, H

Treatment of a copper article in this solution is the same as Example 1but the pH is adjusted to 6.1 and the exposure to the aqueous acidicsolution is for eleven (11) days at which time the outer layer of thecopper has been transformed into malachite.

In Example 5 the composition may be varied in accordance with the tablebelow:

G./l. Potassium chlorate, KClO 2.5-l0 Copper sulfate, CuSO -5H O 0.5-2Sodium bicarbonate, NaHCO 0.5-2

Sulfuric acid, H 80 ing the following composition:

G./l. Potassium chlorate, KClO 2.5-l0 Copper sulfate, CuSO -5H O 0.5-20Sodium sulfate, Na SO Up to 10 Sulfuric acid, H 50 to adjust thesolution to a pH of 2.0-4.0 whereby the cupreous surface is transformedinto brochanite,

3. A method according to claim 1 in which the cupreous surface isexposed for 9-15 days in a solution having the following composition:

G./l. Potassium chlorate, KClO 2.5-l0 Copper sulfate, CuSO -5H O 0.5-2Sodium bicarbonate, NaHCO 0.5-2

Sulfuric acid, H 80 to adjust the solution to a pH of 5.0-6.5 wherebythe cupreous surface is transformed into malachite,

CuCO Cu (OH 2 4. A method according to claim 2 in which the solutioncontains about 5 g./l. of potassium chlorate, about 10 g./l. of coppersulfate and sulfuric acid to adjust the pH to about 3.

5. A method according to claim 2 in which the solution contains about 5g./l. of potassium chlorate, about 3.2 g./l. of copper sulfate, about 5g./l. of sodium sulfate and sulfuric acid to adjust the pH to about 3.

6. A method according to claim 2 in which the solution contains about 5g./l. of potassium chlorate, about 1.25 g./l. of copper sulfate, about 2g./l. of sodium sulfate and sulfuric acid to adjust the pH to about 3.5.

7. A method according to claim 2 in which the solu tion contains about 5g./l. of potassium chlorate, about 1.6 g./l. of copper sulfate, about2.5 g./l. of sodium sulfate and sulfuric acid to adjust the pH to about3.

8. A method according to claim 3 in which the solu- OTHER REFERENCEStion contains about 5 g./l. of potassium chlorate, about Fishlock: Metalcolouring, 1962 Robert Draper 1 g./l. of copper sulfate, about 1 g./1.of sodium bicar- 208 bonate and sulfuric acid to adjust the pH to about6.1.

References Cited 5 RALPH S. KENDALL, Primary Examiner UNITED STATESPATENTS U.S. Cl. X.R.

3,284,249 11/1966 Osborn 1486.24

